The present invention relates to process variable sensors of the type used in the process control industry. More specifically, the invention relates to life expectancy estimation and diagnostics for such a process variable sensor.
Process control transmitters are used to monitor process variables in industrial processes. For example, a transmitter might monitor pressure, temperature or flow (e.g., process variables) and transmit such process variables back to a control room, where a controller sends a control signal back to an actuator (e.g., valve, motor) to control the process. In order to monitor a process variable, the transmitter must include some type of a sensor. For example, transmitters include sensors with resistances or capacitances which vary in response to temperature, deformations or strain which allow the transmitter to measure, for example, temperature, pressure, flow, level, pH or turbidity.
As sensors age or are subjected to harsh environmental conditions, the accuracy of the sensor tends to degrade. Ultimately, the sensor will fail. Diagnostics can be performed on a sensor by monitoring the sensor output signal. For example, a simple diagnostic technique is to compare the sensor output to a maximum or minimum value and provide an alarm indication if the threshold is exceeded. However, one difficulty in prior art diagnostic techniques is that the variations in the process variable being sensed should not be incorrectly interpreted as a sensor fault.